1. Field of the Invention
The invention generally relates to a semiconductor device, and more particularly, to an active device.
2. Description of Related Art
The most popular liquid crystal display (LCD) today is composed of a thin film transistor (TFT) array substrate, a color filter substrate and a liquid crystal layer between the above-mentioned two substrates. On a conventional TFT array substrate, it is mostly adopted that the amorphous silicon (a-Si) TFT or the low-temperature polysilicon TFT servers as a switching element of each sub-pixel. In recent years, some studies have pointed out that in comparison with an a-Si TFT, an oxide semiconductor TFT has higher field-effect mobility; and in comparison with a low-temperature polysilicon TFT, an oxide semiconductor TFT has better threshold-voltage uniformity (Vth uniformity). As a result, the oxide semiconductor TFT has the potential to become a key element for the next generation's planar displays. However, under its existing architecture, the conventional oxide semiconductor TFT is not easy to further enhance field-effect mobility.